P4Se5 has been isolated from the reaction of bromine with P4Se3 in carbon disulphide, and identified by crystal structure determination; the analogous reaction of bromine with P4S3 yields P4S5. The crystals of P4Se5 are orthorhombic, space group Pn21a, a= 12·79(2), b= 11·19(2), c= 6·66(2)Å, Z= 4. The refinement of site occupation factors for trial atoms facilitated the structure determination. The molecule has the same unusual cage structure as P4S5, but the crystals are not isostructural.